Trifluoromethylated aromatic compounds are important intermediates for the production of several commercially important herbicides including triflualinfluometuronnorflurazon. Trifluoromethyl substituted compounds are utilized in other areas as well.
The preparation of trifluoromethylated aromatic compounds by direct trifluoromethylation with trifluoromethyl free radicals is known in the art. This process requires the use of such sources of trifluoromethyl radicals as hexafluoroacetone, trifluoromethyl copper or trifluoromethyl iodide. These reagents waste half of the radicals in forming fluoroform (CF.sub.3 H) and sometimes yield dihydroaromatic dimers as the major product. This process would inherently produce low isomeric selectivity whenever isomer formation is possible. Multiple trifluoromethylation is another problem unless a large excess of aromatic compound is utilized.
U.S. Pat. No. 2,273,992 discloses the process for preparing trifluoromethylnaphthalene utilizing such a free radical process by reacting carbon tetrachloride and anhydrous hydrogen fluoride in the presence of copper. The process disclosed in this patent utilizes a high temperature of 150.degree.-155.degree. C. for extended periods of time (48 hours). Moreover the disclosure in this patent reports the formation of an unspecified amount of by-product containing 41.5% fluorine which is probably C.sub.20 H.sub.12-16 (CF.sub.3).sub.4 resulting from multiple trifluoromethylation and dimerization reactions typical of free radical trifluoromethylations. Cupric fluoride, a possible product of the reaction of copper metal, carbon tetrachloride, and hydrogen fluoride under the conditions of this patent, is a base in liquid hydrogen fluoride.
Trifluoromethylation via coupling of aryl iodides with trifluoromethyl radicals or trifluoromethyl copper reagents reduces the wastage due to formation of CF.sub.3 H. This known process permits isomeric selectivity but requires a troublesome synthesis of the appropriate aryl iodide and recycling of the iodide released. The production of the trifluoromethyl radicals still requires the difficultly obtainable (on a commercial scale) trifluoroacetic acid or trifluoromethyl iodide.
Conversion of carboxyl groups to trifluoromethyl groups is also reported in the literature. However, this process utilizes toxic, expensive and difficultly recycleable reagents such as sulfur tetrafluoride, molybdenum hexafluoride or tungsten pentafluoride. Large quantities of such reagents are required because they, not anhydrous hydrogen fluoride, are the source of fluorine for the conversion.
The most widely utilized process for the preparation of trifluoromethylated aromatic compounds involves the chloride-fluoride exchange reaction using acidic fluorides or hydrogen fluoride as the fluoride source. This reaction is used to produce fluorinated hydrocarbons as well as benzotrifluorides. The reaction can be run without a catalyst at high temperatures but in commercial practice a catalyst such as FeCl.sub.3 or SbCl.sub.2 F.sub.3 is utilized. This process uses readily available raw materials except when an unusual isomer of a methylated aromatic compound is required but takes at least two steps and suffers from corrosion and condensation problems during the exchange step. The chlorination and exchange steps of the process have not been combined because the catalyst for the exchange step also catalyzes nuclear chlorination and because corrosion problems are more difficult to solve when the properties of chlorine are combined with those of hydrogen fluoride - hydrogen chloride mixtures.